The preparation of asymmetric integrally skinned membranes having a defect-free skin is a complicated and tedious task. The presence in the skin layer of pores or defects having a diameter of about 1.0 nm reduces drastically the membrane selectivity. Asymmetric membranes are usually prepared by a phase inversion process as described in U.S. Pat. No. 3,133,132. The permeance or capacity of integrally skinned asymmetric membranes is inversely proportional to the skin thickness when the resistance from the highly porous interior is minimal. Therefore, the skin layer should be as thin as possible, preferably of the order of 100 nm to achieve reasonable permeance or membrane capacity. However, if the skin thickness is reduced, it is more difficult to eliminate the defect pores. Thus, a large proportion of the commercial gas separation membranes used nowadays are composite membranes i.e. they have a thin coating layer applied to an asymmetric support membrane to seal surface pores or defects so as to obtain an adequate selectivity with a suitable capacity. There is still room for improvement in order to produce defect-free integrally skinned membranes.
A forced evaporation method for preparing integrally skinned asymmetric membranes is described in U.S. Pat. No. 4,902,422. In this method, evaporation of a volatile solvent in the casting solution is required at the membrane surface prior to immersion of the nascent membrane into a coagulation bath. This step increases the polymer concentration at the membrane surface and leads to the formation of a defect-free skin. The evaporation period required is long, i.e. 15 to 30 seconds, and limits use in continuous preparation of flat membranes and hollow fibers where the residence time in the air gap is quite short. Another drawback of this method is that it uses volatile organic compounds which have workers health and environmental concerns. Also, since chlorinated hydrocarbons are commonly used as the volatile solvent, water can not be used as the coagulating medium due to mutual immiscibility.
Another method of preparing defect-free asymmetric integrally skinned membranes using common non-volatile solvents is based on a delayed demixing method. For this method, the diffusion induced phase separation, which occurs in the coagulation bath, is delayed. After immersion of a nascent membrane in a non-solvent bath, the outflow of solvent is greater than the inflow of non-solvent and there is a concentration build up at the surface of the membrane. This method was used for the preparation of flat membranes and hollow fibers using a dual-bath coagulation method (J. A. Van't Hof et al., J. Membrane Sci. 70, 17-30, 1992). This method can be used with one single bath composed of two immiscible layers or of two separate baths. For the immiscible layers, there are limitations due to penetration of one liquid layer into the other. For the two bath method, there are limitations due to the fact that the membrane is not coagulated prior to leaving the first bath. It is also difficult to transport the membrane to the second bath without damaging it. This method is also limited in view of the insufficient residence time for the dense skin region to form for continuous casting.
U.S. Pat. No. 5,141,642 also describes an asymmetric integrally skinned membrane having a good gas permeability and selectivity. In particular, this patent describes a delayed demixing method for preparing such membranes, which overcomes some of the limitations of the previous method since skin formation and coagulation occur in the same non solvent bath. The details of skin formation are not divulged in this patent. However, it is known that such a method is very sensitive to the coagulation rate as indicated in U.S. Pat. No. 4,460,526. The polymer/solvent/non-solvent system needs to be carefully chosen and coagulation must be carefully controlled. An example is given for a dry-wet hollow fiber spinning method using a polyimide polymer with chlorophenol solvent and coagulating in a 65:35 by weight ethanol/water mixture. Another drawback inherent to this method is that it requires a solvent exchange drying method to prevent collapse of the transition layer below the skin layer. The membrane is first placed in an ethanol bath to completely remove the coagulating liquid and then, it is placed into a hexane bath to extract ethanol prior to being air dried. The latter steps increase the costs and complexity of membrane preparation.
Polyimides can be divided into two main groups: those which can be dissolved by certain organic solvents and those which can not. Asymmetric membranes may be prepared from both groups. The membranes of the first group can be prepared by direct casting. The membranes of the second group are prepared from a polyimide precursor called a polyamic acid which can be dissolved in organic solvents permitting membrane casting. Subsequent to casting, the polyamic acid membrane is converted to a polyimide membrane either by a thermal treatment or a chemical treatment.
U.S. Pat. No. 4,113,628 describes a method for preparing asymmetric integrally skinned membranes from polyamic acid casting solutions. The casting is done in a non-solvent bath which also chemically converts the polyamic acid to a polyimide. The skin is formed by delayed demixing with a slow coagulation rate. These membranes demonstrated a good gas permeation selectivity for H2 over CH4. However, the H2 permeance is low, which indicates that a relatively thick skin is obtained by using such a method. Such a drawback considerably limits the usefulness of these membranes. A method for the preparation of small flat sheet membranes is also described in this patent. Continuous preparation of large membrane quantities using this method is limited by the costs and complexity of regeneration of the non-solvent reaction bath and the usual problems associated with the delayed demixing method.
Cranford et al. in Journal of Membrane Science 155, (1999), 231-240 disclose polyetherimide/polyvinylpyrrolidone vapor permeation membranes. These membranes are prepared from a mixture of polyetherimide (PEI) and polyvinylpyrrolidone (PVP) according to a wet-phase inversion technique. The polyetherimide is in fact solubilized in an organic solvent such as N-methylpyrrolidone (NMP). However, the membranes described in this document are not solvent resistant.
Huang et al. in Journal of Applied Polymer Science 85, (2002), 139-152 disclose polyimide membranes which can be useful for removal of water from water/organic mixtures. These membranes are prepared by imidizing capillary tubes obtained from a solution including a polyamic acid dissolved in an organic solvent. However, these membranes provided a low selectivity for acetic acid/water. Moreover, the mechanical properties of these membranes were not adequate since the membranes were too brittle and fragile for practical use.
Methods for the preparation of solvent resistant asymmetric microporous membranes are described in U.S. Pat. Nos. 5,725,769 and 5,753,008. The asymmetric membranes prepared according to the latter two patents require a coating layer which provides an adequate selectivity for gas or vapor applications. Unfortunately, such a coating layer increases the cost and renders the production of these membranes more tedious. Also, operational failure may occur due to delamination of the coating layer from the asymmetric support membrane for various reasons such as differences in absorption properties and thermal expansion of the two layers. In many cases, the coating layer limits the range of the operating conditions of the membrane.
Flat dense solvent resistant polyimide membranes prepared from polyamic acids and their salts are described in U.S. Pat. No. 6,497,747. However, the latter patent does not teach or suggest how to prepare asymmetric integrally skinned membranes.